M. Keith Chen, Kareem Haggag, Devin G. Pope, and Ryne Rohla have posted this draft. Here is the abstract:

Equal access to voting is a core feature of democratic government. Using data from millions of smartphone users, we quantify a racial disparity in voting wait times across a nationwide sample of polling places during the 2016 US presidential election. Relative to entirely-white neighborhoods, residents of entirely-black neighborhoods waited 29% longer to vote and were 74% more likely to spend more than 30 minutes at their polling place. This disparity holds when comparing predominantly white and black polling places within the same states and counties, and survives numerous robustness and placebo tests. Our results document large racial differences in voting wait times and demonstrates that geospatial data can be an effective tool to both measure and monitor these disparities.